/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.base;

import static com.google.common.base.NullnessCasts.uncheckedCastNullableTToT;
import static com.google.common.base.Preconditions.checkState;

import com.google.common.annotations.GwtCompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;

import java.util.Iterator;
import java.util.NoSuchElementException;
import javax.annotation.CheckForNull;

import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * Note this class is a copy of {@link com.google.common.collect.AbstractIterator} (for dependency
 * reasons).
 */
@GwtCompatible
@ElementTypesAreNonnullByDefault
abstract class AbstractIterator<T extends @Nullable Object> implements Iterator<T>
{
    private State state = State.NOT_READY;

    protected AbstractIterator()
    {
    }

    private enum State
    {
        READY,
        NOT_READY,
        DONE,
        FAILED,
    }

    @CheckForNull
    private T next;

    @CheckForNull
    protected abstract T computeNext();

    @CanIgnoreReturnValue
    @CheckForNull
    protected final T endOfData()
    {
        state = State.DONE;
        return null;
    }

    @Override
    public final boolean hasNext()
    {
        checkState(state != State.FAILED);
        switch (state)
        {
            case DONE:
                return false;
            case READY:
                return true;
            default:
        }
        return tryToComputeNext();
    }

    private boolean tryToComputeNext()
    {
        state = State.FAILED; // temporary pessimism
        next = computeNext();
        if (state != State.DONE)
        {
            state = State.READY;
            return true;
        }
        return false;
    }

    @Override
    @ParametricNullness
    public final T next()
    {
        if (!hasNext())
        {
            throw new NoSuchElementException();
        }
        state = State.NOT_READY;
        // Safe because hasNext() ensures that tryToComputeNext() has put a T into `next`.
        T result = uncheckedCastNullableTToT(next);
        next = null;
        return result;
    }

    @Override
    public final void remove()
    {
        throw new UnsupportedOperationException();
    }
}
